Moto M: Phone in "Red State". Does not boot. Cannot enter fastboot or recovery

I tried to install a custom ROM on my Moto M, and failed in the attempt. I uploaded TWRP, and misguidedly executed adb flashing lock afterwards. Now my phone says:

Red State.
Your device has failed verification and may not work properly.
Your device will boot in 5 seconds.

But it does not boot. The screen simply goes black, and reboots to the same screen. I can enter the "meta mode", but I don't know what use that is, because neither in the normal mode nor in the meta mode the phone recognized by adb or fastboot. Linux still recognizes the phone a some sort of USB device, though:

usb 3-3: new high-speed USB device number 2 using xhci_hcd
usb 3-3: new full-speed USB device number 3 using xhci_hcd
usb 3-3: New USB device found, idVendor=0e8d, idProduct=2000
usb 3-3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 3-3: Product: MT65xx Preloader
usb 3-3: Manufacturer: MediaTek
cdc_acm 3-3:1.1: ttyACM0: USB ACM device
usbcore: registered new interface driver cdc_acm
cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
usb 3-3: USB disconnect, device number 3

Pressing volume down + power button does nothing. The phone simply remains off. If I keep pressing the power button a long time the phone vibrates, but still does not boot. I have no I idea what I can do. Did I brick my Moto M? I don't really care anymore about custom ROMs or stock ROMs, I simply want my phone to work again at all. :( Please tell me if you have any ideas or suggestions!